Not stock firmware. Difficult process to flash to stock firmware.
This phone comes with special firmware that only works on Verizon. To use this with 3CX, for instance, you will need to flash it with stock firmware. This is not a simple process, but it is possible by putting the phone into recovery mode. The phone supports restores from a TFTP server, which is a pretty antiquated process.
I would suggest that you pay a bit more and get a phone that already has stock firmware installed, to save you the hassle of setting up a TFTP server and reading up on this process.